An image processor for an automotive vehicle is disclosed in, for example, Japanese Patent No. 3444192. The processor detects a white line (i.e., a lane line) on a traffic road on the basis of an image of the road in front of an automotive vehicle. The image of the road is taken by an imaging element. On the basis of sharpness of an edge of the white line, distance distribution of unsharpness of each part of the white line is calculated. The distance distribution of unsharpness is compared with a reference distance distribution of unsharpness, which is preliminarily determined to be a reference under a predetermined reference visibility. Thus, a ratio between the detected distance distribution and the reference distance distribution is obtained as a distance distribution of a ratio of unsharpness. The distance distribution of the ratio of unsharpness is approximated with a straight line. The image processor estimates environment around the imaging element on the basis of the slope of the approximate straight line, dispersion of the distance distribution of the ratio of unsharpness with respect to the approximate straight line, time change of the approximate straight line, and average of the ratio of unsharpness.
Thus, conventionally, the imaging environment is determined on the basis of the approximate straight line, which is obtained by approximating the distance distribution of ratio of unsharpness. Therefore, visibility reduction caused by bad weather such as rain and fog is detectable by using the image processor. However, the image processor cannot determine reduction of reliability for imaging the white line. The reduction of reliability for imaging the white line is caused by a factor other than weather. Specifically, for example, there is an area, in which the white line is not visible (i.e., detectable) by the imaging element, because the area is displaced apart from the imaging element and it is in the nighttime. A part of the white line is interrupted by other vehicles and/or other bodies, so that the part of the white line is not detected by the imaging element. Thus, when the distance distribution is approximated by the straight line, the image processing device may handle as if a part of the white line, which is not actually detected by the imaging element, is captured.